Hi all, RR here. I have been asked to try and fix a "1 hour" ni-cad charger for an 18 volt battery drill and I was appalled at what I found inside. There is just a transformer feeding a bridge rectifier made from four 1N54xx diodes and a 47 ohm resistor which is shorted out to start with by relay contacts which are triggered to open and place the resistor in series when a temperature sensor in the battery pack opens and triggers the relay. So there is no current limiting at all, just raw unsmoothed and uncontrolled DC feeding direct to a pack of 15 ni-cad cells, and of course what happens is that some of the cells short out and therefore remain cold and this causes more current to flow through the pack and heat it up even more and what I've sometimes found with crude charging systems like this is it's often the cell(s) nearest to the sensor that short out so then there is no longer any temperature monitoring of the remaining working cells so the rest of the pack can heat up to a point where it could explode! Isn't it about time the relevant authorities cracked down on stuff like this? Don't ni-cad and ni-mh cells need constant current controlled by some kind of current limiter, preferably with a regulator of some sort and not just a resistor as this doesn't keep the current limited enough if some cells short out? RR.

Sure it's a Ni-Cad? A lot of modern drills use Li-ion batteries and the charging protection is built in to the battery....I know because I have some in for test at work. There is a PCB inside the battery and effectively this controls the charging current.

I suspect that some of the earlier batteries mostly used with brand name laptops had a cycle counter that switched the output terminals off until the battery was fully discharged. This would usually take many weeks and force the owner to buy a new battery.The reset procedure involved cooling the battery to a few degrees below the minimum temperature shown on the battery casing.

Hi, from RR. Yep, it's definitely a ni-cad, it's marked on it. And I've had the battery open and there's no PCB, just a little sensor that's normally closed in series with the cell negative which opens when it heats up. The positive contact goes direct to the cell positive and the negative contact goes through the sensor ( which is dead short until it opens ) to the cell negative. The cell negative goes to a third contact through which the charging current returns once the sensor opens and this triggers the relay which opens it's NC contacts to put the 47R resistor in series with the battery. The fault was no power due to a failed thermal fuse in the transformer most likely caused by shorted out cells caused by such dreadfully crude "charging". The transformer survived... RR.

Refugee wrote:I suspect that some of the earlier batteries mostly used with brand name laptops had a cycle counter that switched the output terminals off until the battery was fully discharged. This would usually take many weeks and force the owner to buy a new battery.The reset procedure involved cooling the battery to a few degrees below the minimum temperature shown on the battery casing.

Forgive me Ref, but what are you on about and what relevance does it have to the OP's question...?

I did some time ago open up an early NiMh battery that had come from an early laptop and it did have a board in it with quite a few chips and a couple of power FETs and an overload sensor.The battery in question had nothing wrong with the cells after being stored for 10 years and I reused them to good effect with a 14 hour charger.Those thermostatic chargers with a relay and resistor were used on the first Nicad powered portable VHS recorders.